The installation of flexible supporting and sealing liners is described in U.S. Pat. Nos. 5,176,207, 5,803,666, and 6,026,900. In some cases, it is desirable to temporarily insert a rigid liner within a cylindrical cavity, such as a borehole, and to then replace the rigid liner with a flexible inflated liner with contact between the inflated liner and the cylindrical cavity during the replacement procedure.
However, if one inflates a flexible liner inside a rigid pipe of the same or less diameter, the flexible liner can not be easily withdrawn from the pipe, nor can the pipe be easily removed from off the flexible liner because of the excessive drag of the inflated flexible liner on the rigid liner. The pressure of the inflated flexible liner against the pipe wall and the large surface area of the pipe combine to produce very large drag resistance to sliding the pipe off the flexible liner. For example, a differential pressure of 1 psi in a 10 ft. long flexible liner inside a 4 in. diameter pipe, with a drag coefficient of 1.0, requires over 1500 lbs of force to pull the inflated liner out of the pipe. Such a large force may tear the flexible liner. As the pipe length increases to that of common well depths, the larger drag force is certain to tear the flexible liner.
It is common practice to emplace a rigid liner (e.g., a pipe) to support a drill hole wall while drilling a well. It may then be useful to replace the rigid liner support of the hole wall with the support of an inflated flexible liner. However, if the inflated liner is anchored to the bottom of the hole, and one pulls upward on the rigid liner, the drag of the rigid liner on the inflated liner prevents the rigid liner from being removed from the hole without a potentially destructive drag force on the flexible liner.
The present invention eliminates most of the drag force of a rigid liner on the inflated liner without the use of a lubricant to reduce the friction coefficient.
Various objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.